FinFETs with low source/drain contact resistance

ABSTRACT

An integrated circuit structure includes a semiconductor substrate, insulation regions extending into the semiconductor substrate, with the insulation regions including first top surfaces and second top surfaces lower than the first top surfaces, a semiconductor fin over the first top surfaces of the insulation regions, a gate stack on a top surface and sidewalls of the semiconductor fin, and a source/drain region on a side of the gate stack. The source/drain region includes a first portion having opposite sidewalls that are substantially parallel to each other, with the first portion being lower than the first top surfaces and higher than the second top surfaces of the insulation regions, and a second portion over the first portion, with the second portion being wider than the first portion.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/229,218, filed on Mar. 28, 2014, and entitled “FinFETs with LowSource/drain Contact Resistance,” which application is herebyincorporated herein by reference.

BACKGROUND

Transistors typically include semiconductor regions that are used toform the source regions and drain regions. The contact resistancebetween metal contact plugs and the semiconductor regions is high.Accordingly, metal silicides are formed on the surfaces of thesemiconductor regions such as silicon regions, germanium regions, andsilicon germanium regions in order to reduce the contact resistance. Thecontact plugs are formed to contact the silicide regions, and thecontact resistance between the contact plugs and the silicide regions islow.

A typical silicidation process includes forming a metal layer on thesurfaces of the semiconductor regions, and then performing an annealing,so that the metal layer reacts with the semiconductor regions to formthe silicide regions. After the reaction, the upper portions of themetal layer may be left un-reacted. An etching step is then performed toremove the un-reacted portions of the metal layer. Contact plugs arethen formed to contact the silicide regions.

With the increasing down-sizing of integrated circuits, the silicideregions, and hence the contact between the contact plugs and thesilicide regions, also become increasingly smaller. Accordingly, thecontact resistance of the electrical contacts becomes increasinglyhigher. For example, in Fin Field-Effect Transistors (FinFETs), the finsare very narrow, causing the contact areas between the contact plugs andthe fins to be very small. The contact resistance to the source anddrain regions of the FinFETs thus becomes an increasingly severeproblem.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIGS. 1 through 11C are cross-sectional views and perspective views ofintermediate stages in the manufacturing of a Fin Field-EffectTransistor (FinFET) in accordance with some exemplary embodiments; and

FIGS. 12 and 13 are cross-sectional views of FinFETs in accordance withalternative embodiments.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The following disclosure provides many different embodiments, orexamples, for implementing different features of the invention. Specificexamples of components and arrangements are described below to simplifythe present disclosure. These are, of course, merely examples and arenot intended to be limiting. For example, the formation of a firstfeature over or on a second feature in the description that follows mayinclude embodiments in which the first and second features are formed indirect contact, and may also include embodiments in which additionalfeatures may be formed between the first and second features, such thatthe first and second features may not be in direct contact. In addition,the present disclosure may repeat reference numerals and/or letters inthe various examples. This repetition is for the purpose of simplicityand clarity and does not in itself dictate a relationship between thevarious embodiments and/or configurations discussed.

Further, spatially relative terms, such as “underlying,” “below,”“lower,” “overlying,” “upper” and the like, may be used herein for easeof description to describe one element or feature's relationship toanother element(s) or feature(s) as illustrated in the figures. Thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the figures. The apparatus may be otherwiseoriented (rotated 90 degrees or at other orientations) and the spatiallyrelative descriptors used herein may likewise be interpretedaccordingly.

A Fin Field-Effect Transistor (FinFET) and the method of forming thesame are provided in accordance with various exemplary embodiments. Theintermediate stages of forming the FinFET are illustrated. Theintermediate stages of forming contacts to the FinFET are alsoillustrated. The variations of the embodiments are discussed. Throughoutthe various views and illustrative embodiments, like reference numbersare used to designate like elements.

FIGS. 1 through 11C are cross-sectional views and perspective views ofintermediate stages in the manufacturing of a FinFET and the respectivecontacts in accordance with some exemplary embodiments. FIG. 1illustrates a perspective view of an initial structure. The initialstructure includes wafer 100, which further includes substrate 20.Substrate 20 may be a semiconductor substrate, which may further be asilicon substrate, a silicon germanium substrate, or a substrate formedof other semiconductor materials. Substrate 20 may be doped with ap-type or an n-type impurity. Isolation regions 22 such as ShallowTrench Isolation (STI) regions may be formed to extend from a topsurface of substrate 20 into substrate 20, wherein the top surface ofsubstrate 20 is a major surface 100A of wafer 100. The portions ofsubstrate 20 between neighboring STI regions 22 are referred to assemiconductor strips 24. The top surfaces of semiconductor strips 24 andthe top surfaces of STI regions 22 may be substantially level with eachother.

STI regions 22 may include silicon oxide, which may be formed using, forexample, High-Density Plasma (HDP) Chemical Vapor Deposition (CVD). STIregions 22 may also include an oxide formed of Flowable Chemical VaporDeposition (FCVD), spin-on, or the like.

Referring to FIG. 2, STI regions 22 are recessed, so that top portionsof semiconductor strips 24 are higher than the top surfaces of STIregions 22 to form semiconductor fins 24′. The etching may be performedin a dry etching process, wherein HF and NH₃ are used as the etchinggases. In alternative embodiments, the etching gases include NF₃ andNH₃. During the etching process, plasma may be generated. Alternatively,in the etching process, plasma is turned on. In an exemplary etchingprocess, the etching gases have a pressure in the range between about100 mtorr and about 200 mtorr. The flow rate of HF may be in the rangebetween about 50 sccm and about 150 sccm. The flow rate of NH₃ may be inthe range between about 50 sccm and about 150 sccm. Argon may also beincluded, with a flow rate in the range between about 20 sccm and about100 sccm. In alternative embodiments, the recessing of STI regions 22 isperformed using wet etching. The etchant may include diluted HF, forexample.

Referring to FIG. 3, gate stack 29 is formed on the top surface andsidewalls of semiconductor fins 24′. Gate stack 29 includes gatedielectric 27, and gate electrode 26 over gate dielectric 27. Gateelectrode 26 may be formed, for example, using polysilicon, althoughother materials such as metal silicides, metal nitrides, or the like,may also be used. Gate stack 29 may also comprise a hard mask layer (notshown) over gate electrode 26, wherein the hard mask layer may comprisesilicon oxide, for example. Gate stack 29 may cross over a single one ora plurality of semiconductor fins 24′ and/or STI regions 22. Gate stack29 may also have a lengthwise direction substantially perpendicular tothe lengthwise direction of semiconductor fins 24′. In some embodiments,gate stack 29 forms the gate stack of the resulting FinFET. Inalternative embodiments, gate stack 29 is a dummy gate stack, and willbe replaced by a replacement gate in a subsequent step.

Next, gate spacers 28 are formed on the sidewalls of gate stack 29. Insome embodiments, gate spacers 28 comprise silicon carbonitride (SiCN),silicon nitride, or the like, and may have a single-layer structure or amulti-layer structure.

An etching step (referred to as source/drain recessing hereinafter) isthen performed to etch portions of semiconductor fins 24′ that are notcovered by gate stack 29 and gate spacers 28, resulting in the structureshown in FIG. 4A. The recessing may be anisotropic, and hence theportions of semiconductor fins 24 directly underlying gate stack 29 andgate spacers 28 are protected, and are not etched. The top surfaces 24Aof the recessed semiconductor strips 24 are lower than the top surfaces22A of STI regions 22. Recesses 31 are accordingly formed between STIregions 22. Recesses 31 are located on opposite sides of gate stack 29.

FIG. 4B illustrates a structure in accordance with alternativeembodiments of the present disclosure, in which the source/drainrecessing is performed until recesses 31 extend to a level below thebottom surfaces of STI regions 22. When recesses 31 reach the bottomsurfaces of STI regions 22, further recessing will cause recesses 31 toexpand laterally since there is no sidewalls of STI regions 32preventing the lateral expansion. Accordingly, recesses 31 have theprofile as shown in FIG. 4B.

Next, as shown in FIG. 5, epitaxy regions 30 are formed by selectivelygrowing a semiconductor material in recesses 31. In some exemplaryembodiments, epitaxy regions 30 comprise silicon germanium or silicon.Depending on whether the resulting FinFET is a p-type FinFET or ann-type FinFET, a p-type or an n-type impurity may be in-situ doped withthe proceeding of the epitaxy. For example, when the resulting FinFET isa p-type FinFET, SiGeB may be grown. Conversely, when the resultingFinFET is an n-type FinFET, SiP may be grown. In alternativeembodiments, epitaxy regions 30 comprise III-V compound semiconductorssuch as GaAs, InP, GaN, InGaAs, InAlAs, GaSb, AlSb, AlAs, AlP, GaP,combinations thereof, or multi-layers thereof. After recesses 31 arefilled with epitaxy regions 30, the further epitaxial growth of epitaxyregions 30 causes epitaxy regions 30 to expand horizontally, and facetsmay start to form. Furthermore, some of top surfaces 22A of STI regions22 are underlying and aligned to portions of epitaxy regions 30 due tothe lateral growth of epitaxy regions 30.

After the epitaxy step, epitaxy regions 30 may be further implanted witha p-type or an n-type impurity to form source and drain regions, whichare also denoted using reference numeral 30. In alternative embodiments,the implantation step is skipped since source and drain regions areformed during the epitaxy due to the in-situ doping of the p-type orn-type impurity. Source and drain regions 30 are on opposite sides ofgate stack 29, and may be overlying and overlapping portions of surfaces22A of STI regions 22. Epitaxy regions 30 include lower portions 30Athat are formed in STI regions 22, and upper portions 30B that areformed over the top surfaces 22A of STI regions 22. Lower portions 30A,whose sidewalls are shaped by the shapes of recesses 31 (FIG. 4), mayhave (substantially) straight edges, which may also be vertical edgesthat are perpendicular to the major surfaces (a bottom surface 20B) ofsubstrate 20 as being shown in FIG. 5. For example, the tilt angle θ ofthe sidewalls of lower portions 30A may be in the range between about 80degrees and about 90 degrees.

FIG. 6 illustrates a perspective view of the structure after Inter-LayerDielectric (ILD) 36 is formed. In some embodiments, a buffer oxide layer(not shown) and a Contact Etch Stop Layer (CESL) are formed on sourceand drain regions 30 before the formation of ILD 36. In someembodiments, the buffer oxide layer comprises silicon oxide, and theCESL may comprise silicon nitride, silicon carbonitride, or the like.The buffer oxide layer and the CESL may be formed using Atomic LayerDeposition (ALD), for example. ILD 36 may comprise Flowable oxide formedusing, for example Flowable Chemical Vapor Deposition (FCVD). ILD 36 mayalso include Phospho-Silicate glass (PSG), Boro-Silicate Glass (BSG),Boron-Doped Phospho-Silicate Glass (BPSG), Tetra Ethyl Ortho Silicate(TEOS) oxide, or the like. A Chemical Mechanical Polish (CMP) may beperformed to level the top surfaces of ILD 36, gate stack 29, and gatespacers 28 with each other.

Next, dummy gate and dummy gate have to be replaced with HK and MG, thennext the portions 36A of ILD 36 are removed to form contact openings.One of the contact openings 38 is shown in FIG. 7. FIGS. 7 through 10are cross-sectional views obtained from the same vertical planecontaining line A-A in FIG. 6.

As shown in FIG. 7, epitaxy regions 30 are exposed to contact openings38. The buffer layer and the CESL, if any, will be removed from contactopenings 38 in order to expose epitaxy regions 30. Contact opening 38 islocated in ILD 36. Source/drain regions 30 may include a plurality ofspade-shaped (diamond-shaped) epitaxy regions separated from each otherin accordance with some embodiments. Epitaxy regions 30 have facets 30Cand 30D. Facets 30C are upward facing facets and facets 30D are downwardfacing facets. Facets 30C and 30D may be on <111> planes of epitaxyregions 30.

Next, referring to FIG. 8, STI regions 22 that are exposed throughcontact opening 38 are recessed using an etching step. The etching maybe performed in a dry etching process. In some embodiments, the etchinggases include HF and NH₃. In alternative embodiments, the etching gasesinclude NF₃ and NH₃. During the etching process, plasma may begenerated. Alternatively, in the etching process, plasma is generated.In an exemplary etching process, the etching gases have a pressure inthe range between about 100 mtorr and about 200 mtorr. The flow rate ofHF may be in the range between about 50 sccm and about 150 sccm. Theflow rate of NH₃ may be in the range between about 50 sccm and about 150sccm. Argon may also be included, with a flow rate in the range betweenabout 20 sccm and about 100 sccm.

As shown in FIG. 8, after the STI recessing, the recessed top surfaces22B of STI regions are below the level 41, at which portions 30B ofsource/drain regions 30 join the respective underlying portions 30A ofsource/drain regions 30. Accordingly, the sidewalls of epitaxysemiconductor portions 30A are exposed. As a result of the STIrecessing, STI regions 22 include top surfaces 22A and top surfaces 22Bthat are lower than top surfaces 22A.

FIG. 9 illustrates the formation of source/drain silicide regions 44 onthe exposed surfaces of source/drain regions 30. The formation ofsource/drain silicide regions 44 include forming a conformal metal layer(not shown) in opening 38, wherein the conformal layer is deposited onthe exposed surfaces of portions 30A and 30B of source/drain regions 30.The metal layer might use a conformal deposition method such as AtomicLayer Deposition (ALD). The metal layer may include titanium, nickel,cobalt, or the like. An annealing is performed. In accordance with someembodiments, the annealing is performed using, for example, thermalsoaking, spike annealing, flash annealing, laser annealing, or the liketo form the metal silicide regions 44. Throughout the description, theterms “metal silicide” and “metal silicide/germanide” are used asgeneric terms to refer to metal silicides, metal germanides, and metalsilicon germanides. The unreacted portions of the metal layer are thenremoved.

Source/drain silicide regions 44 are formed on the sidewalls of portion30A, wherein the sidewalls of portions 30A are on opposite sides ofportions 30A, and the opposite sidewalls of portions 30A aresubstantially parallel to each other, and are substantially vertical.Source/drain silicide regions 44 are further formed on the surfaces ofportions 30B, which are laterally expanded beyond the edges of therespective underlying portion 30A.

FIG. 10 illustrates the filling of opening 38 (FIG. 9) with a conductivematerial. After the filling of the conductive material, a ChemicalMechanical Polish (CMP) is performed to remove the excess portion of theconductive material, and the remaining conductive material in opening 38forms contact plug 42. In some embodiments, contact plug 42 comprisestungsten. In alternative embodiments, contact plug 42 comprises othermetal(s) or metal alloys such as aluminum, copper, titanium nitride,tantalum nitride, or the like. The filling of opening 38 may beperformed using ALD, with the conductive material comprising tungsten,for example. The precursor may include WF₆, for example. Contact plug 42extends from the top surface of ILD 36 to contact top surfaces 22B ofSTI regions 22.

It is appreciated that although source/drain portions 30B as in FIG. 10have the shape of diamonds, they may also have other cross-sectionalshapes due to the formation process and the subsequent annealingprocesses. For example, the corners of source/drain portions 30B may bemuch more rounded than illustrated.

FIG. 11A illustrates a perspective view in the formation of replacementgate. First, dummy gate dielectric 27 and dummy gate electrode as shownin FIG. 6 are removed. A gate dielectric layer and a gate electrodelayer may then be formed to fill the openings left by the removed dummygates, followed by a CMP to remove excess portions of the gatedielectric layer and the gate electrode layer. The remaining replacementgates include gate dielectric 50 and gate electrode 52. Gate dielectric50 may comprise a high-k dielectric material with a k value greater thanabout 7.0, for example, and gate electrode 52 may comprise a metal or ametal alloy. Gate dielectric 50, gate electrode 52, and source and drainregions 30 in combination form FinFET 54.

FIG. 11B illustrates the cross-sectional view of FinFET 54, wherein thecross-sectional view is obtained from the plane crossing line B-B inFIG. 11A. The top surface 22A (also refer to FIG. 4A) and 22B (alsorefer to FIGS. 4A and 8) of STI regions 22 are illustrated. Thepositions of fins 24′ and silicide regions 44 are also illustrated. Asclearly shown in FIG. 11B, silicide regions 44 and contact plug 42extend below the bottom of semiconductor fin 24′.

FIG. 11C illustrate the cross-sectional view of FinFET 54, wherein thecross-sectional view is obtained from the plane crossing line C-C inFIG. 11A. For simplicity, one fin 24′ is formed, although there is aplurality of fins 24′ in the cross-sectional view. As shown in FIG. 11C,semiconductor fin 24′ is above the top surface 22A of STI regions 22.Gate dielectric 50 and gate electrode 52 are formed on the top surfaceand sidewalls of semiconductor fin 24′. The recessed top surface 22B ofSTI regions 22 is thus lower than the bottom of semiconductor fin 24′.

Referring back to FIG. 10 again, the vertical distance from the top endof epitaxy regions 30 to the bottom ends of STI regions 22 is defined asbeing length L1. The vertical distance from the bottom surface ofepitaxy regions 30 to the bottom surfaces of STI regions 22 are definedas length L2. The vertical distance from the top end of source/drainregions 30 to the top surfaces 22A of STI regions 22 is defined as beinglength L3. In some exemplary embodiments, length L1 may be in the rangebetween about 80 nm and about 200 nm. Length L3 may be in the rangebetween about 20 nm and about 100 nm. It is appreciated, however, thatthe values recited throughout the description are merely examples, andmay be changed to different values. In accordance with some embodiments,there is the relationship L1>L3>L2. Also, the ratio L3/L1 may be in therange between about 20 percent and about 100 percent. It is observedthat by increasing the length L3, the sidewalls of portions 30A ofsource/drain regions 30 are exposed to form silicide regions 44, andhence the contact area is increased. The source/drain contact resistanceis thus reduced due to the increased contact area.

FIG. 12 illustrates the cross-sectional views of epitaxy regions 30, STIregions 22, silicide regions 44, and contact plug 42 in accordance withalternative embodiments. These embodiments are similar to theembodiments in FIGS. 11A, 11B, and 11C, except that epitaxy regions 30extend to a level below the bottom surfaces of STI regions 22. TheFormation process includes the step shown in FIG. 4B, wherein during therecessing of semiconductor strips 24, recesses 31 extend below thebottom surfaces of STI regions 22. The rest of the processes areessentially the same as what are shown in FIGS. 1 through 11C. In theseembodiments, ratio L3/L1 may be in the range between about 20 percentand about 100 percent. In some exemplary embodiments, length L1 may bein the range between about 80 nm and about 200 nm. Length L3 may be inthe range between about 20 nm and about 100 nm.

FIG. 13 illustrates the cross-sectional views of epitaxy regions 30, STIregions 22, silicide regions 44, and contact plug 42 in accordance withyet alternative embodiments. These embodiments are similar to theembodiments in FIG. 12, except that after the step as shown in FIG. 7,the portions of STI regions 22 that are exposed to opening 38 will befully etched. Hence, no STI region exists between neighboringsource/drain regions 30. Silicide regions 44 will be formed on the topsurfaces 20A of substrate 20. In these embodiments, length L1 is equalto length L3.

The embodiments of the present disclosure have some advantageousfeatures. By recessing STI regions after the epitaxy step for formingthe epitaxy source/drain regions, the sidewalls of lower portions of thesource/drain regions are exposed. As a result, the source/drain contactarea is increased, and the source/drain contact resistance is reduced.

In accordance with some embodiments of the present disclosure, anintegrated circuit structure includes a semiconductor substrate,insulation regions extending into the semiconductor substrate, with theinsulation regions including first top surfaces and second top surfaceslower than the first top surfaces, a semiconductor fin over the firsttop surfaces of the insulation regions, a gate stack on a top surfaceand sidewalls of the semiconductor fin, and a source/drain region on aside of the gate stack. The source/drain region includes a first portionhaving opposite sidewalls that are substantially parallel to each other,with the first portion being lower than the first top surfaces andhigher than the second top surfaces of the insulation regions, and asecond portion over the first portion, with the second portion beingwider than the first portion.

In accordance with alternative embodiments of the present disclosure, anintegrated circuit structure includes a semiconductor substrate,insulation regions extending into the semiconductor substrate, with theinsulation regions having a first top surface, and a first semiconductorfin and a second semiconductor fin parallel to each other and spacedapart from each other by a first portion of the insulation regions. Thefirst portion of the insulation regions has a first top surface. Theintegrated circuit structure further includes a first source/drainregion and a second source/drain region connected to the firstsemiconductor fin and the second semiconductor fin, respectively. Asecond portion of the insulation regions is between the firstsource/drain region and the second source/drain region. The secondportion of the insulation regions has a second top surface lower thanthe first top surface.

In accordance with yet alternative embodiments of the presentdisclosure, a method includes forming a semiconductor fin, wherein thesemiconductor fin is over top surfaces of insulation regions, with theinsulation regions being on opposite sides of the semiconductor fin,forming a gate stack on a top surface and sidewalls of a middle portionof the semiconductor fin, etching an end portion of the semiconductorfin to form a recess, wherein the recess extends between oppositeportions of the insulation regions, and performing an epitaxy to grow anepitaxy semiconductor region. The epitaxy semiconductor region includesa first portion in the recess, and a second portion over the topsurfaces of the insulation regions. After the epitaxy, the insulationregions are etched. After the etching the insulation regions, a contactplug is formed to electrically couple to the epitaxy semiconductorregion.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A method comprising: forming a semiconductor fin,wherein the semiconductor fin is over top surfaces of isolation regions,with the isolation regions being on opposite sides of the semiconductorfin; forming a gate stack on a top surface and sidewalls of a firstportion of the semiconductor fin; etching a second portion of thesemiconductor fin to form a recess, wherein the recess extends betweenopposite portions of the isolation regions; performing an epitaxy togrow an epitaxy semiconductor region, wherein the epitaxy semiconductorregion comprises a first portion in the recess, and a second portionover the top surfaces of the isolation regions, wherein the firstportion of the epitaxy semiconductor region comprises opposite sidewallssubstantially parallel to each other; after the epitaxy, etching theisolation regions, wherein the opposite sidewalls of the epitaxysemiconductor region are exposed after the etching the isolationregions; and after the etching the isolation regions, forming a contactplug electrically coupling to the epitaxy semiconductor region.
 2. Themethod of claim 1 further comprising, before the forming the contactplug, forming a silicide region on sidewalls of the first portion andthe second portion of the epitaxy semiconductor region.
 3. The method ofclaim 2, wherein the second portion of the epitaxy semiconductor regionhas a down-facing facet, and the silicide region extends onto thedown-facing facet.
 4. The method of claim 1, wherein the recess extendsto a level higher than bottom surfaces of the isolation regions.
 5. Themethod of claim 1, wherein a top surface of the isolation regionsrevealed after the etching the isolation regions is higher than a bottomsurface of the epitaxy semiconductor region.
 6. The method of claim 1further comprising: after the epitaxy, forming an Inter-Layer Dielectric(ILD) over the epitaxy semiconductor region; and etching the ILD to forma contact opening, wherein the second portion of the semiconductor finis exposed through the contact opening, and the etching the isolationregions is performed through the contact opening.
 7. The method of claim1, wherein the etching the isolation regions comprises an anisotropicetching.
 8. A method comprising: performing a first recessing to recessisolation regions, wherein a semiconductor region between the recessedisolation regions forms a semiconductor fin protruding higher than firsttop surfaces of the isolation regions; forming a gate stack covering afirst portion of the semiconductor fin; etching a second portion of thesemiconductor fin to form a recess in the isolation regions; performingan epitaxy to grow an epitaxy semiconductor region from the recess;performing a second recessing to recess the isolation regions, so thatsidewalls of the epitaxy semiconductor region in contact with therecessed isolation regions are exposed, wherein after the secondrecessing, a bottom portion of the epitaxy semiconductor region has asidewall contacting a remaining portion of the isolation regions; andperforming a silicidation on the epitaxy semiconductor region to form asource/drain silicide region.
 9. The method of claim 8 furthercomprising: after the epitaxy, forming an Inter-Layer Dielectric (ILD)covering the epitaxy semiconductor region; and etching the ILD to form acontact opening, wherein the epitaxy semiconductor region and portionsof the isolation regions are exposed through the opening, and the secondrecessing is performed on the exposed portions of the isolation regions.10. The method of claim 9, wherein after the second recessing, asidewall of the isolation regions is flushed with an edge of the ILD,with the edge of the ILD being an edge inside the contact opening. 11.The method of claim 8, wherein before the second recessing, the epitaxysemiconductor region comprises a bottom portion having sidewallscontacting the isolation regions, and a top portion higher than theisolation regions, with the top portion comprising up-facing anddown-facing facets.
 12. The method of claim 11, wherein the source/drainsilicide region is formed on surfaces of both the up-facing anddown-facing facets of the top portion of the epitaxy semiconductorregion, and the source/drain silicide region is further formed on thebottom portion of the epitaxy semiconductor region.
 13. The method ofclaim 8, wherein the second recessing comprises a dry etching.
 14. Amethod comprising: forming a gate stack covering a first portion of asemiconductor fin, wherein the semiconductor fin protrudes higher thantop surfaces of isolation regions on opposite sides of the semiconductorfin, and the gate stack has a bottom surface contacting a first portionof the top surfaces of the isolation regions, with a second portion ofthe top surfaces of the isolation regions is exposed; forming asource/drain region on a side of the gate stack; forming an Inter-LayerDielectric (ILD) to cover the source/drain region; etching the ILD toform a contact opening, wherein the source/drain region and the secondportion of the top surfaces of the isolation regions are revealedthrough the contact opening; and etching a portion of the isolationregions that has the second portion of the top surfaces through thecontact opening.
 15. The method of claim 14 further comprisingperforming a silicidation on the source/drain region to form asource/drain silicide region.
 16. The method of claim 14, wherein afterthe recessing the isolation regions through the contact opening,portions of sidewalls of the source/drain region in contact with theetched portions of the isolation regions are revealed.
 17. The method ofclaim 14, wherein after the recessing, a sidewall of the isolationregions is flushed with an edge of the ILD, with the edge of the ILDbeing an edge inside the contact opening.
 18. The method of claim 14,wherein the recessing the isolation regions through the contact openingcomprises a dry etch.
 19. The method of claim 1, wherein after theportion of the isolation regions is etched, the isolation regions has atop surface lower than an entirety of the gate stack.
 20. The method ofclaim 14, wherein before the portion of the isolation regions is etched,a first top surface of the portion of the isolation regions issubstantially level with a bottom surface of the gate stack, and afterthe portion of the isolation regions is etched, a second top surface ofthe isolation regions is lower than an entirety of the gate stack.